A characteristic property required of separator paper for isolating a positive electrode active material and a negative electrode active material in an alkaline-battery, is the ability to prevent internal shortages caused by touching of both electrode active materials or caused by an acicula (dendrite) of zinc oxide which is produced by discharge of the negative electrode. Another characteristic property is for separator paper to have a desirable durability, insofar as the separator paper desirably does not undergo shrinkage and deformation in the presence of electrolyte such as potassium hydroxide and the presence of positive electrode active material such as manganese dioxide, nickel oxy-hydroxide or silver oxide. At the same time, the separator paper should not block the ionic conduction.
Generally, separator paper for alkaline batteries uses a mixed fiber paper consisting of synthetic fiber and cellulose fiber. More specifically, the mixed fiber paper includes vinylon fiber of alkali proof synthetic fiber as the main material, viscose rayon fiber, linter pulp, mercerized wood pulp, and polynosic rayon fiber each of which is alkali proof cellulose fiber, and polyvinyl alcohol fiber added as a binder. On making the separator paper, the cellulose fiber such as the linter pulp, the mercerized wood pulp, and the polynosic rayon fiber, is subjected to beating treatment as needed, and fine fibril is produced from a fiber body, in order to give high fineness to the separator paper.
Although separator paper made by interweaving the synthetic fibers with the cellulose fibers has sufficient durability against the electrolyte and the active material, the separator paper has a pore size which is sufficiently large to create problems of internal shortages caused by touching of the active materials with one another or caused by the dendrite of zinc oxide which is produced from negative electrode. In order to overcome the above-mentioned problem of internal shortages, it is therefore required to cylindrically laminate the separator papers, each of which has a thickness of 100 μm, in triple or quadruple layers (thickness of 300 μm or 400 μm), on installing the separator papers in the alkaline battery. This measure of covering one separator paper with other separator papers makes the effective pore size smaller. In addition, a conditioner (inhibitor), which prevents growth of dendrite of zinc oxide, may be added to the electrolyte in order to prevent an internal shortage from occurring. Furthermore, it is required to overlap the separator paper with a separator material such as a cellophane film having ionic permeability and high shielding property.
The present applicant has disclosed techniques for preventing internal shortage in alkaline batteries using separator paper, in Japanese Unexamined Patent Publication Tokkai Hei 2-119049 and 10-92411 (hereinafter “JP 2-119049” and “JP 10-92411”, respectively). The separator paper disclosed in JP 2-119049 is made by interweaving the synthetic fibers with the alkali proof cellulose fibers applicable for beating, such as the mercerized wood pulp or the polynosic rayon and includes alkali proof cellulose fiber falling within 10 to 50 weight %, and has a beating degree with the alkali proof cellulose fiber falling within a 500 to 0 ml range of CSF (Canadian Standard Freeness) value.
The separator paper disclosed in JP 10-92411 is made by lamination of dense layer and liquid keeping layer. The dense layer is made by interweaving the synthetic fibers with the alkali proof cellulose fibers applicable for beating and includes alkali proof cellulose fiber falling within 20 to 80 weight %, and has a beating degree of the alkali proof cellulose fiber falling within a 500 to 0 ml range of CSF value. The liquid keeping layer is made by interweaving the synthetic fibers with the alkali proof cellulose fibers and includes alkali proof cellulose fiber falling within 20 to 80 weight %, and has a beating degree of the alkali proof cellulose fiber which is not less than 700 ml of CSF value.
The above-described separator paper is made by laminating in triple or quadruple layers in order to prevent the internal shortage, on installing the separator paper in the alkaline battery. As a result, the installed separator paper has a thickness of about 300 μm.
When the number of laminated separator papers increases in the alkaline battery, the volume occupied by the separator paper increases in the alkaline battery. As a result, the available space for (and hence the amount of) positive electrode active material and negative electrode active material is reduced, thereby reducing the electric capacity of battery. Furthermore, the distance between the positive electrode and the negative electrode becomes longer as the number of laminated separator papers increases. When the distance between the electrodes becomes longer, the internal resistance of battery becomes large. Because voltage drop is based on the increase of internal resistance, the electric capacity of battery further reduces. In addition, high rate discharge having high electric current becomes poor when the internal resistance becomes large.
Under the circumstances, it is desirable to develop a thin separator paper having high gas tightness and high shielding property that is capable of occupying a reduced volume in the alkaline battery and preventing internal shortage, in order to further improve the electric properties such as increase of active materials, improvement of high rate discharge, increase of electric capacity, and reduction of internal resistance.
When the separator paper becomes thin, it is possible to reduce the internal resistance of the alkaline battery and it is possible to increase the electric capacity of the battery inasmuch as the available space for and the amount of active materials in the battery increase. However, the risk of internal shortages occurring in the battery increases when the separator paper is thin. More particularly, the produced dendrite (e.g., zinc oxide) grows from the negative electrode to the positive electrode, causing an internal shortage when the dendrite reaches to the positive electrode. The shortage occurs in a short time when using the thin separator paper having low shielding.
The above-mentioned JP 2-119049 is intended for preventing the internal shortage based on the dendrite of zinc oxide which occurs with low mercury of the negative electrode. However, the alkali proof cellulose fibers applicable for beating is not greater in amount than 50 weight % in the separator paper disclosed in JP 2-119049 and the separator paper disclosed therein has low shielding. When the separator paper has a thickness less than 300 μm in a dry condition, such as after being used in the battery, it is difficult to sufficiently prevent the internal shortage.
The above-mentioned JP 10-92411 is intended for preventing the internal shortage based on the dendrite of alkaline battery and improving heavy discharge (high rate discharge) property. In the obtained separator paper, the liquid keeping layer greatly expands in the electrolyte and keeps a great amount of electrolyte therein. As a result, it is possible to improve the high rate discharge property of the battery. However, it is necessary to reduce the amount of active materials inasmuch as the separator paper consequently occupies a large area in the battery when the separator paper expands in the battery. Although the high rate discharge property is improved, the electric capacity reduces in a low rate discharge.
In the above separator papers, natural cellulose pulp, which is a basic ingredient of mercerized pulp, has the crystalline structure of cellulose 1. On the other hand, the mercerized pulp having an improved alkali proof has the crystalline structure of cellulose 2.
For obtaining the improved alkali proof mercerized pulp, the wood such as coniferous tree and broad-leaved tree or non-wood such as Cotton linter and Manila hemp is subjected to a cooking treatment, in order to obtain the natural cellulose pulp. After the natural cellulose pulp is subjected to an immersing treatment in NaOH aqueous solution having concentration of 18 weight % to 25 weight %, the alkaline solution is removed from the natural cellulose pulp by water washing, in order to obtain the improved alkali proof mercerized pulp. In other words, a low molecular weight component such as hemicelluloses is dissolved and removed from the natural cellulose pulp having the crystalline structure of cellulose 1, by mercerizing treatment, in order to refine the cellulose. The natural cellulose pulp varies from the crystalline structure of cellulose 1 to alkaline cellulose. By removing the alkali solution by water washing, the natural cellulose pulp irreversibly varies from the alkaline cellulose to the crystalline structure of cellulose 2. The cellulose 2 has a crystalline structure of high alkali proof cellulose.
So, it is possible to reduce the solubility of the separator paper and to reduce size shrinkage of the separator paper in the electrolyte when the mercerized pulp is used as the separator paper for an alkaline battery. Accordingly, it is known that the shielding property of the separator paper is improved, when the mercerized pulp is subjected to beating treatment in order to fibrillate the mercerized pulp. The present applicant has offered the separator paper derived from the mercerized pulp, which is the alkali proof cellulose fiber capable of beating and which is highly beaten, in order to enhance gas tightness.
However, the present applicant has found it is difficult to fibrillate the mercerized pulp having the structure of cellulose 2 by the beating treatment. It is possible to easily beat non-mercerized pulp composed of the cellulose 1 till obtaining 0 ml in CSF value. On the other hand, the fibril's structure is strongly combined with other fibrils in the pulp fiber having the perfect crystalline structure of cellulose 2, and the fibrils are hardly produced by the beating treatment. Even if the fibrils are produced, the produced fibrils are easily cut off. As a result, it takes a long time to beat the pulp having the perfect cellulose 2. It is possible to carry out the beating treatment with respect to the pulp having the crystalline structure of cellulose 2 by mercerizing, although it takes a long time to beat the pulp having the cellulose 2 till 100 ml of CSF value. However, the produced fibrils and the pulp fibers are remarkably cut off when continuing the beating treatment. More specifically, it is difficult to reduce CSF value by the beating treatment in a region in which the CSF value is not greater than 50 ml. In this condition, new fibrils are hardly produced from the pulp fibers by the beating treatment. The pulp fibers and the fibrils are cut off and only the fiber length becomes short.
Even if the beaten pulp in this condition has a small CSF value which is representative of degree of beating, it is difficult to interweave high gas tightness separator paper by using the beaten pulp as raw material. The fibril produced by beating is cut off by continued beating and the fibril becomes a plurality of fine fiber pieces. As a result, the fine fiber pieces flow out through wire cloth of paper making and pinholes occur on the sheet. Under the circumstances, the tensile strength of the separator paper reduces and a plurality of pinholes exists in the separator paper. Accordingly, the pin holes in the separator paper cause an internal shortage of the electrode active materials to occur when the separation paper includes only a single layer or double layers, or when the thickness is not greater than 120 μm.
For example, the mercerized pulp, the linter pulp, pre-hydride pulp, the polynosic fiber, or the like are included as the alkali proof cellulose fiber capable of beating, in the above-mentioned Japanese patent publications. However, each of the mercerized pulp and polynosic fiber has the structure of perfect cellulose 2. It is difficult to carry out the beating treatment of cellulose 2 in high beating region of 50 ml to 0 ml of CSF value, as will be described hereinafter. Furthermore, it is difficult to sufficiently enhance gas tightness and sufficiently prevent internal shortage in a battery when the separator paper of the battery has a thickness which is not greater than about 120 μm. The above-mentioned problem is caused by the separator paper thickness of about 100 μm and the optimum gas tightness range of 2 seconds/100 ml to 100 seconds/100 ml, which are described in JP 10-92411.
Furthermore, the above-described separator paper made by interweaving the cellulose fiber with the synthetic fiber has insufficient endurance for prolonged use. In case of using regenerated cellulose fiber such as rayon fiber, polynosic fiber, solvent spun rayon fiber, or the like, the regenerated cellulose fiber deteriorates when contacted with the positive electrode active material over a long duration, inasmuch as the degree of polymerization of regenerated cellulose fiber is low in value, such as 200 to 600. As a result, the part dissolving in the electrolyte increases in the regenerated cellulose fiber. More particularly, the fine fiber portion which is fibrillated is easily dissolved. Inasmuch as the fine fiber portion selectively disappears, the shielding property of the separator paper is reduced in use. Therefore, there is a problem in which the internal shortage easily occurs in the battery.
On the other hand, the crystals of the dendrite become smaller in case of adding the inhibitor. As a result, the time at which the dendrite reaches the positive electrode becomes longer and the shortage rarely occurs in the battery. Although the addition of inhibitor restrains the growth of dendrite, the addition of inhibitor blocks the ionization of zinc in the negative electrode. Accordingly, the inhibitor reduces the discharging property of the battery.
It is also possible to use cellophane to prevent contact between the active materials and the resulting internal shortage based on the growth of dendrite. However, the separator paper has very high gas tightness and the movement of ions is restrained inasmuch as the cellophane has density of about 1.4 g/m3. As a result, there is a problem in which the internal resistance becomes large in the battery. Furthermore, it is not possible to carry out the high rate discharge in the battery, because the cellophane has a large internal resistance.
The design of positive and negative electrodes has been reviewed in the alkaline battery recently. It is planned to promote the diffusion of ions on the surface of active materials and to improve the high rate discharge property in the alkaline battery by increasing the electrolytic contents of both positive and negative electrodes or increasing the surface area of each of the active materials. Therefore, a thin separator paper is required which has high shortage prevention effect, in order to improve the battery property by shortening the distance between the positive and negative electrodes and by reducing the electric resistance.